Such ring magnets can be manufactured in various ways, for example from a rubber-like material into which hard-ferromagnetic particles are embedded. This rubber-like material is then mounted in a yoke of this kind. Such ring magnets are also referred to as “rubber magnets.”
There are also ring magnets that are manufactured by injection molding from a raw granulated plastic, for example from SmFeN with a polyamide “PA 12” matrix, and, in this case, a slip agent that contains silicone molecules is often added to the raw granulate. Such ring magnets are referred to hereinafter as “plastic-matrix ring magnets.”
With this type of plastic-matrix ring magnet having added silicone, adhesive bonding is not reliable in processing terms, i.e. the adhesive compound cross-links with the yoke, but as a result of contaminants such as, for example    silicone residues,    release agents,    oils,    fats, etc.that are present on the side of the plastic-matrix ring magnet that is to be adhesively bonded, insufficient cross-linking, of the adhesive compound with the ring magnet at that location, is possible in some circumstances, and the risk therefore exists that the relative position of the ring magnet and rotor housing may change in the context of further process steps, e.g. under thermal or mechanical loads.
For this reason, laborious cleaning processes are necessary prior to adhesive bonding in order to increase process reliability. In addition, such cleaning processes clean the surface that is to be bonded only for a certain time. Subsequent diffusion of silicone molecules degrade adhesion capability not only directly prior to bonding, but also during the time when the adhesive compound is curing.
Thus, whereas the adhesive compound reliably cross-links with the yoke, such cross-linking on the surface of a plastic-matrix ring magnet of this kind is not possible in every case to a sufficient extent, as a result of the aforementioned contaminants.